Press brake for bending sheets

ABSTRACT

The present invention relates to a press brake for bending metal sheets wherein:
         at least one stopper is disposed in each slot, each stopper comprising a first wedge having a first end secured to the first slot edge and a second end forming a first surface, and a second wedge having a first end secured to the second slot edge and a second end forming a first surface; and   the first surface of at least one of the first and second wedges has a central portion that is domed or protuberant relative to the other portions of said surface so that the contact between the first and the second wedges is established essentially over said central portion.

FIELD OF THE INVENTION

The present invention relates to a bending press or “press brake” havingtables with controlled deformation.

BACKGROUND OF THE INVENTION

Bending presses are machine tools of a type that is itself well known.As shown in accompanying FIG. 1, the machine tool comprises a lowertable 12 and an upper table 14 that is movable relative to the lowertable 12. Usually, the lower table 12 is stationary and the upper table14 is suitable for being moved towards the lower table 12 under drivefrom actuators V₁ and V₂ that act on the ends 14 a and 14 b of the uppertable 14. Usually, the lower table 12 has its free edge 12 a fitted withfastener means 16 for fastening bending matrices 18. In the same way,the edge 14 c of the upper table 14 is fitted with fastener means 20 forfastening bending punches 22.

A metal sheet or lamination F is placed on the bending matrices 18 ofthe lower table 12. The sheet F may be of a length that varies widelydepending on the circumstances. Under drive from the pistons of theactuators V₁ and V₂, the punches 22 mounted on the upper table 14 movetowards the sheet placed on the matrices of the lower table. As soon asthe punch 22 comes into contact with the sheet F, force begins toincrease within the sheet as the punch penetrates therein, initially inthe elastic range and subsequently in the plastic range, therebyenabling the sheet to be bent permanently.

Because the force is applied to the upper table by the actuators V₁ andV₂ acting on the ends of the table, the linear load distributed betweenthe two ends of the tables corresponds to the upper table being deformedalong a line in the form of a concave arc with deformation maximas closeto the midplane of the table. This means that, for bending purposes, atthe end of bending, the central portions of the punches have penetratedinto the sheet less than have the end portions. If bending were to beperformed on a matrix that, itself, were to remain perfectly straightduring bending, then the result would be that a workpiece would beobtained having a bend angle that was wider in its central portion thanat its ends. Such a result is naturally unacceptable.

In order to remedy that drawback, various solutions have been proposedfor the purpose of controlling these deformations at the edges of thetables by using various means in order to obtain a bend that issubstantially identical over the entire length of the bent workpiece.

Conventionally, these solutions involve providing slots, such as theslots 24 and 26 shown in FIG. 1, that are formed in the lower table 12symmetrically about the midplane P′P of the press. These slots 24, 26then define a central zone 28 of the lower table 12 that is slot-freeand that presents a length b, each of the two slots 24 and 26 being oflength a.

With slots 24 and 26 of conventional type, i.e. that leave between thema slot-free portion 28 of length b, substantially parallel deformationsare obtained for the edges of the upper and lower tables 14 and 12. Thisensures that proper bending is achieved. Nevertheless, this result isobtained only when the metal lamination or sheet F for bending has alength that is substantially equal to the total length of the lower orupper tables 12 or 14. In contrast, when the length of the sheet F isless than the total length of the lower or upper table 12 or 14, both ofthe deformations of the lower and upper tables 12 and 14 are concave.

In addition to the difficulty of proposing a bending press that issuitable for enabling the metal lamination or sheet F for bending to bedeformed substantially uniformly over the entire length of saidlamination or sheet F, regardless of whether its length is shortcompared with the length of the tables 12, 14 of the press or, on thecontrary, is equal to the length of the tables 12, 14 of the press,there exists an additional difficulty related to the top edges 24″, 26″of the slots 24, 26 deforming while the bending force of the movingtable 14 is being applied to the stationary table 12, and said force isbeing taken up on the bottom edges 24′, 26′ of the slots 24, 26.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to remedy these two problems byproposing to dispose at least one stopper in each of the slots 24, 26,which stopper is made up of two elements or wedges that have firstsurfaces fastened to respective ones of the edges of the slots, andsecond surfaces adapted for localized mutual contact substantially inthe centers of the wedges, in a manner such as to ensure excellenttransmission of the bending force from the top edges of the slots to thebottom edges of the slots.

The invention thus provides a press brake for bending at least one metalsheet, said press brake comprising:

-   -   an upper table having a bottom edge carrying first bending        tools, and a lower table having a top edge carrying second        bending tools, the two tables being movable relative to each        other to exert a bending force on the sheet;

said press brake presenting a vertical midplane, one of said tableshaving, through its entire thickness, two slots disposed symmetricallyabout the midplane, each slot having a first edge and a second edge, andan open first end opening out in a side edge of the table, as well as aclosed end;

wherein:

at least one stopper is disposed in each slot, each stopper comprising afirst wedge having a first end secured to the first slot edge and asecond end forming a first surface, and a second wedge having a firstend secured to the second slot edge and a second end forming a firstsurface; and

the first surface of at least one of the first and second wedges has acentral portion that is domed or protuberant relative to the otherportions of said surface so that the contact between the first and thesecond wedges is established essentially over said central portion.

The expression “secured to the first/second edge” is used to mean thatthe wedge in question is connected to the first or second edge, it beingunderstood that said wedge may be movable relative to said edge of theslot.

Other characteristics of the press brake of the invention are indicatedbelow:

-   -   advantageously, the first surface of the first wedge and the        first surface of the second wedge both have respective central        portions that are domed or protuberant relative to the other        portions of said first surfaces;    -   in an embodiment, the first surface of the first wedge and/or        the first surface of the second wedge is a convex surface;    -   in an embodiment of the invention, the first surface of one of        the wedges presents a concave surface while the first surface of        the other wedge presents a convex surface;    -   in an embodiment of the invention, the first surface of at least        the first wedge and/or of the second wedge is a spherical        surface portion;    -   advantageously, at least in the zone of the stoppers, the slots        present constant height so that the first edge and the second        edge are parallel, in the absence of bending force for bending        the metal sheet F;    -   advantageously, the first surfaces of the wedges are inclined        relative to the parallel edges of the slots;    -   preferably, the first surfaces of the wedges are inclined at a        slope lying in the range 1% to 40% and preferably in the range        5% to 10%, relative to an axis or a plane that is parallel to        the edges of the slots;    -   in an embodiment of the invention, the first surface of at least        the first wedge or of the second wedge presents a plurality of        inclined plane peripheral portions connecting to the central        portion;    -   the central portion presents a height or protrusion lying in the        range 0.05 millimeters (mm) to 0.25 mm relative to the other        portions of the first surface for a wedge of length        substantially equal to 80 mm;    -   advantageously, the wedges are mounted on supports connected to        respective ones of the edges of the slots; at least one of the        supports is movable laterally, i.e. along an axis parallel to        the parallel edges of the slot on which it is mounted;    -   in a possibility offered by the invention, the two wedges are        offset relative to each other laterally, i.e. along an axis        parallel to the parallel edges of the slot;    -   in a possibility offered by the invention, in the absence of        bending force for bending the metal sheet F, the first and        second wedges present clearance between them; and    -   advantageously, the press brake of the invention has a plurality        of stoppers disposed in respective ones of the slots        symmetrically about the midplane P′P.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear moreclearly on reading the following description of preferred embodiments ofthe invention given by way of non-limiting example. The descriptionrefers to the accompanying drawings, in which:

FIG. 1 shows a press brake having two slots situated on respective sidesof the midplane P′P and extending from opposite sides;

FIG. 2 is a diagrammatic view showing an embodiment of a stopper that ismade up of two wedges of the invention, one of the wedges being fastenedto the top edge of a slot 24 or 26, and the other being fastened to thebottom edge of the slot 24 or 26;

FIG. 3 is a diagrammatic view of two stoppers provided with amotor-driven control system for controlling the value of the clearanceassociated with the stoppers;

FIGS. 4 to 7 are diagrammatic views of embodiments of a stopper of theinvention, made up of two wedges that are initially in mutual contact;

FIG. 8 is a side view of a stopper wedge of the invention;

FIGS. 9 to 12 show various embodiments of the first surface or contactsurface of a stopper wedge of the invention;

FIG. 13 diagrammatically shows the force fields or pressure fieldspassing through the wedges of a stopper of the invention, when a bendingforce is applied to the metal sheet F causing contact and a force F₀between the wedges;

FIG. 14 is a perspective view of a wedge whose first surface or contactsurface is made up of three portions; and

FIG. 15 is a side view in section of two wedges, one below the other,that are identical to the wedge of FIG. 14.

MORE DETAILED DESCRIPTION

FIG. 2 is a section view of two wedges 28, 29 of a stopper 27 of theinvention. The two wedges 28, 29 have respective contact first surfaces28′, 29′ that, prior to application of a bending force F₀, presentclearance j between them, as also shown in FIGS. 5 and 15.

Each of the wedges 28, 29 is mounted on a respective support 40, 41connected to a respective edge 24′, 26′ or 24″, 26″ of a respective oneof the slots 24 and 26. The function of each wedge 28, 28′, 29,29′/stopper 27 is to control the extent to which the edges 24′, 24″ and26′, 26″ of each slot 24, 26 move towards each other when the bendingforce is applied. By controlling the extent to which the edges 24′, 24″and 26′, 26″ of the slot 24 or 26 move towards each other, it ispossible to control the deformation of the top edge 24″, 26″ of the slot24, 26, and therefore the deformation of the top edge 12 a of the lowertable 12.

At least one of the supports 40 or 41, and optionally both of thesupports 40 and 41 is/are mounted to move laterally, i.e. along an axisparallel to the parallel edges 24′, 26′ and 24″, 26″ of the slots 24, 26on which it is mounted. In the example shown to illustrate the inventionin FIGS. 2 and 3, only the supports 40 are suitable for being moved bymeans of the set of actuators 60, the supports 41 being fastened to theslot edges 24″, 26″ of the lower table 12. Said supports 40 are moved bymeans of a set of actuators 60, shown in FIG. 3, which set is connectedvia link arms 61 to the moving supports 10. The set of actuators 60 iscontrolled by a remote control unit (not shown in the accompanyingfigures). The motor-driven movement of the supports 40, and thus of thewedges 29, makes it possible to adjust the position of at least one ofthe wedges 29 relative to the wedge 28, with a view to applying thebending force F₀. This adjustment defines the value of the clearance j,it being understood that the initial adjustment of the relativepositions of the wedges 28, 29 (before the bending force F₀) is appliedmay also not make provision for any clearance, so that the wedges 28, 29are in abutment with each other. The clearance j between the wedges 28,29 or the relative position of the wedges 28, 29 can be adjusted towithin one hundredth of a millimeter by means of the set of actuators60.

Naturally, it is possible to make provision for the set of actuators 60,or for a distinct drive mechanism, also to enable the supports 41 andthus the wedges 28 to move. By way of example, FIG. 8 shows a wedge 28,29 in which tapped orifices 71, 72, 73 can be seen (the orifices 71, 72are seen end-on, while the orifice 73, situated in an adjacent face, isshown in dashed lines in the thickness of the wedge), the orifices beingdesigned to enable the wedge 28 or 29 to be fastened to a moving support40 or to a stationary support 41 by conventional mechanical means, suchas a screw or a threaded rod.

In order to allow only limited lateral movement, the supports 40 areprovided with slots or holes 62 that extend linearly to define the axesalong which the moving supports 40 move, inside which slots guide pins63 are disposed that are adapted to fit said slots 62. The movement ofthe supports 40 and of the wedges 29 is ideally parallel to the edges24′, 26′ of the slots 24, 26. It should be noted that, advantageously,the edges 24′, 24″, and 26′, 26″ of each of the slots 24, 26 areparallel, at least at the wedges 28, 29/stoppers 27.

The first surface 28′, 29′ of each of the wedges 28, 29 advantageouslyhas an inclination relative to the axes or to the planes of the paralleledges 24′, 26′ and 24″, 26″ of the slots 24, 26. This inclination of thefirst surface 28′, 29′ of each of the wedges 28, 29 lies in the range 1%to 30%, as a function of the material forming the wedges 28, 29, or moreexactly of the coefficient of friction of the materials used toconstitute the contact surface 28′, 29′ of each of the wedges 28, 29.Thus, by way of example, it can be noted that the inclination of each ofthe wedges 28, 29 shown in FIGS. 2 and 3 lies in the range 2% to 10%while said inclination lies in the range 10% to 30% for the wedges 28,29 shown, for example, in FIG. 6, 13, or 14. It should also be notedthat the inclinations of the wedges 28, 29 coming into abutment witheach other may be the same or slightly different.

In an essential aspect of the invention, at least one of the firstsurfaces, or contact surfaces, 28′, 29′ of the respective wedges 28, 29is provided with a domed or protuberant central portion 30, 31 so thatthe contact between the first and the second wedges 28 and 29 isestablished essentially over said central portion 30 or 31.

This domed or protuberant central portion 30, 31 may be of variousshapes and may be present on one of the two wedges 28 or 29 only, or onboth of the wedges 28, 29. In addition, as explained below through thevarious embodiments, due to the shape of the domed or protuberantportion 30, 31 of each of the first surfaces 28′, 29′ of the wedges 28,29, the contact between the wedges 28, 29 may consist of contact at apoint or substantially at a point, of contact along a line, or ofcontact over an area.

In FIG. 4, each of the wedges 28 and 29 has a domed or protuberantcentral portion 30, 31, which portions form the contact zone of the twowedges 28, 29. In this example, the bottom wedge 29 is situated closerto the open end 26 a of the slot 26 than is the top wedge 28 so thatthere is a slight lateral offset between the two wedges 28, 29. In thisexample, the first surface 28′, 29′ of each of the two wedges 28, 29consists of a spherical surface, but the vertex S of the sphericalsurface of each of the two wedges 28, 29 is not situated exactly in thecenter of the first surface 28′ or 29′. That is why the two wedges 28,29 are offset slightly laterally relative to each other so that thecontact between the two wedges, initially and/or while the bending forceF₀ for bending the sheet F is being applied, makes contact possible atthe central portions 30, 31. Naturally, this manner of arranging thewedges 28, 29 relative to each other is a function of the sphericalsurfaces of the two first surfaces 28′, 29′ of the wedges 28, 29, but itis also a function of the flexing of the top portion 12 c of the lowertable 12, and thus of the extent to which the top wedge 28 itself moves.

In general, it should be noted that, by convention, the vertex of thedomed or protuberant central portion 30, 31 is considered relative to aplane P₀ joining two opposite edges 80, 81 of the wedge 28 or 29, whichplane P₀ corresponds to the inclination of the wedge 28, 29. The vertexS is the point of the domed or protuberant central portion 30, 31 thatis at the furthest distance (“protrusion”) from the plane P₀. This planeP₀ is shown in FIGS. 14 and 15 that show a last embodiment of theinvention. As can be seen in FIG. 14, the plane P₀ is the plane joiningthe two opposite edges 80, 81. The vertex S of the domed or protuberantcentral portion 30, 31 may be located at one end of said portion, asshown in FIG. 14. The maximum height of the vertex S is referenced h. InFIG. 15, it can be noted that substantially the entire domed orprotuberant central portion 30, 31 is at the height h relative to theplane P₀. It can also be understood that, due to the inclination of thefirst surfaces 28′, 29′ of the wedges 28, 29, the vertex S of theprotuberant portion 30, 31 does not necessarily coincide with the pointof the first surface 28′, 29′ that is furthest away from the slot edge26′, 26″ to which the wedge 28, 29 is fastened.

FIG. 5 shows a variant of FIG. 4. In this example, the bottom wedge 29is situated further away from the open end 26 a than is the top wedge28, so that, once again, there is a slight lateral offset between thetwo wedges 28 and 29, but the offset is the reverse of the lateraloffset of the wedges 28, 29 shown in FIG. 4. In addition, in the initialstate, in the absence of any force exerted by the actuators V₁, V₂, thefirst surfaces 28′, 29′ present clearance j between them. In theembodiment of FIG. 5, as in the embodiment of FIG. 4, the arrangement ofthe domed central portion 30, 31 of each of the first surfaces 28′, 29′of each of the wedges 28, 29 and the relative lateral offset of the twowedges 28, 29 are chosen so that, while the bending force is beingapplied to the metal sheet F, the two wedges 28, 29 are in contact overtheir respective domed or protuberant central portions 30, 31.

FIGS. 6 and 7 show the bending force F₀ for bending the metal sheet Fthat essentially causes the top portion 12 c of the lower table 12 toflex so that the top edge 26″ of the slot 26 moves closer to the bottomedge 26′ of the same slot 26.

Once again, in these two FIGS. 6 and 7, the wedges 28 and 29 are offsetlaterally relative to each other. In the embodiment of FIG. 6, onlywedge 29 has a first surface 29′ with a domed or protuberant centralportion 31, e.g. a spherical surface or a plane protuberant surface. Thecontact between the two wedges 28, 29 takes place, for the wedge 29,over its domed or protuberant portion 31. It should be noted that, whenonly one of the two wedges 28 or 29 has a domed or protuberant centralportion 30, 31, it is advantageous for the wedge that is provided withthe central portion 31 to be the bottom wedge 29 that is fastened to thebottom edge 26′ of the slot 26. The wedges 28, 29 with their protuberantcentral portions 30 and/or 31 and their relative lateral offset aredesigned to compensate for the non-parallelism of the edges 24′, 24″ and26′, 26″ of the slots 24 and 26.

The embodiment of FIG. 7 is analogous to the embodiment shown in FIG. 4,but FIG. 7 shows the top portion 12 c of the lower table flexing whilethe bending force F₀ is being applied, and said force being transmittedto the lower table 12.

In addition to the tapped orifices 71, 72, and 73 serving to fasten thewedge 28 or 29 to a support 40, 41, FIG. 8 shows a first surface 28′,29′ seen from the side, on which surface it is difficult to see with thenaked eye that there is a domed or protuberant central portion 30, 31.This is because the first surface 28′, 29′ of the wedge 28, 29 is aspherical surface that has a radius of curvature that is extremely largerelative to the length of the wedge 28, 29. By way of example, the wedge28, 29 of FIG. 8 has a length lying in the range 60 mm to 80 mm and theradius of curvature of each of the first surfaces 28′, 29′ lies in therange 7000 mm (or 7 meters) to 9000 mm (or 9 meters). The protrusion,i.e. the maximum height of the central portion 30, 31 relative to theplane surface of the first surface, which plane surface is defined bythe straight line joining the opposite ends 80, 81 of the first surface28′, 29′, lies approximately in the range 0.05 mm to 0.25 mm. The domedor protuberant portion 30, 31 thus has a maximum height or protrusionlying in the range 0.05% of the length (longest surface dimension) ofthe first surface 28′, 29′ to 0.4% of said length (when the inclinationof the first surface is not too large, it can be considered, byapproximation, that the length of each of the wedges is equal to thelength of its first surface), said maximum height or protrusionpreferably lying in the range 0.1% of the length of the first surface28′, 29′ of the wedge 28, 29 to 0.3% of said length. It can beunderstood that the difference in height of the domed or protuberantcentral portion 30, 31 is often not perceptible with the naked eye, andthat the accompanying figures intentionally magnify the central portion30, 31 for reasons of simplification and of understanding.

FIG. 9 shows a wedge 28, 29 having a first surface 28′, 29′ that issubstantially cylindrical and dished. In this example, the first surface28′, 29′ of the wedge 28, 29 is provided with a dome constituting thedomed or protuberant central portion 30, 31 of the wedge 28, 29. Thisdome may consist of a spherical surface that is protuberant relative tothe first surface 28′, 29′ presenting a section of substantiallycylindrical shape.

The wedge 28, 29 shown in FIG. 10 has an inclined cylindrical firstsurface 28′, 29′. The center O of the sphere, of which the first surface28′, 29′ of the wedge 28 or 29 forms a portion, is offset relative tothe vertical V starting from the center C of the first surface 28′, 29′(this vertical V intersects the bottom plane formed by the first endsecured to the first edge of the slot 24′ or 26′). In this example, thecentre C of the first surface 28′ or 29′ of the wedge 28 or 29constitutes the point of contact with the first surface 28′ or 29′ ofthe other wedge 28 or 29. Thus, when the first surface 28′ or 29′ has aspherical surface, the contact with the first surface 28′, 29′ of theother wedge 28 or 29, regardless of the shape thereof, is contact at apoint or substantially at a point. The area of contact 28′, 29′ betweenthe two wedges 28, 29 is thus very small and, taking account of themanufacturing tolerances for the wedges 28, 29, and of the materialsused, it represents about 1 square millimeter (mm²). In general manner,this point or substantially point contact between the two wedges 28, 29may optionally take place at the center C of each of the first surfaces28′, 29′ of the wedges 28, 29 but, as explained above, the contactbetween the two wedges 28, 29 depends on their respective inclinationsand on their relative offset, as well as on the movement of the topwedge 28 while the bending force F₀ for bending the metal sheet F isbeing applied.

FIG. 11 shows a variant embodiment of the first surface 28′, 29′ of thewedge 28, 29. In this embodiment, the central portion 30, 31 consists ofa plane surface. This central portion 30, 31 is in the form of arectangular or square surface representing in the range 5% of the totalarea of the first surface 28′, 29′ to 25% of said total area, andpreferably in the range 10% of said total area to 15% thereof. In thisexample, the first surface 28′, 29′ of the wedge 28, 29 has fourinclined plane peripheral portions 33, 34, 35, and 36 extending fromrespective ones of the four edges of the first surface 28′, 29′ to thecentral portion 30, 31. In this example in which the central portion 30,31 is a plane surface, the contact with the first surface 28′, 29′ ofthe other wedge 28, 29, presenting a plane contact surface (optionallythe first surface 28′, 29′ of said other wedge 28, 29 is identical tothe first surface of the wedge 28, 29 shown in FIG. 11), is area contactbetween the two wedges 28 and 29.

FIGS. 14 and 15 also show an embodiment in which the contact between thefirst two surfaces 28′, 29′ of the two wedges 28, 29 is area contact. Asshown in these figures, the first contact surface 28′, 29′ issubstantially an inclined surface, i.e. the opposite edges 80, 81 havedifferent heights. In addition, the first surface 28′, 29′ of the wedges28, 29 has three successive segments 40, 30 or 31, and 42, extendingover the entire width of the wedge 28, 29, and each having a differentinclination; the inclinations of the segments 40, 30/31, and 42increasing going from the segment 40 to the segment 42. The intermediateor central segment constitutes the central or protuberant portion 30,31. Relative to the plane or the axis 50 joining the opposite edges 80,81 of the wedge 28, 29, the central portion 30, 31 presents a maximumheight h of about 0.1 mm. As can be seen in FIG. 15, the two wedges 28,29 that are offset slightly relative to each other are identical but theorientations of their respective first surfaces 28′, 29′ are opposite sothat only the central portions 30, 31 face each other and aresubstantially parallel. Because of the offset between the two wedges 28,29, only a portion of each of the central portions 30, 31 comes intocontact, of the area contact type, with the respective portion of theother central portion. It should be noted that, in this example, the twowedges 28, 29 present clearance j in the initial state.

FIG. 12 shows the third possible mode of contact between the two wedges28, 29, namely linear contact, the first two modes being point orsubstantially point contact, and area contact. In this example that ischosen to illustrate this third type of contact, only the wedge 28 has aprotuberant central portion 31. In this example, the first surfaces 28′,29′ of the wedges 28, 29 are cylindrical surfaces, but while the firstsurface 28′ lies on the inside of a cylinder so that the first surface28′ is protuberant relative to the plane/axis joining the opposite edgesof the wedge 28, the first surface 29′ lies on the outside of a cylinderso that the first surface 29′ forms a recess relative to the plane/axisjoining the opposite edges 80, 81.

In addition, the center O₁ of the cylinder on which the first surface28′ lies is closer to said surface 28′ than the center O₂ of thecylinder on which the first surface 29′ lies. Thus, the radius of thecylinder of which the first surface 28′ forms a portion is smaller thanthe radius of the cylinder of which the first surface 29′ forms aportion. That is why only the vertex of the central portion 30 of thefirst surface 28′ comes into contact over the entire width of the firstsurface 29′ of the wedge 29, so that the contact between the two wedges28, 29 is linear contact.

FIG. 13 shows the force lines that are exerted while the bending forceF₀ for bending the metal sheet F is being applied. The force linesconverge or are concentrated from the first end of the wedge 28 that isfastened to the edge 26″ of the slot 26 towards the protuberant centralportion 30 of the first surface 28′ of the wedge 28 that is in contactwith the protuberant central portion 31 of the first surface 29′ of thewedge 29; these force lines then spread out over the entire width of thewedge 29. In this example, the first surfaces 28′, 29′ of the wedges 28,29 present spherical or cylindrical surfaces so that the contact isrespectively point contact, substantially point contact, or linearcontact. The wedges 28, 29 may be made of hardened steel while the lowertable 12 may be made of mild steel, thereby, in the absence of plasticdeformation, making it possible for stress to be high between the wedges28, 29 but low between the wedges 28, 29 and the lower table 12.

Naturally, the contact between the first surfaces 28′, 29′ that isdescribed as being point contact or linear contact is the first contactduring or at the beginning of application of the force F₀ because, afterthis point or linear contact, the pressure from the top wedge 28 on thebottom wedge 29 is such that the first surfaces 28′, 29′ of the wedges28, 29 enter at least an elastic deformation stage so that a contactzone that is larger is reached. While the force F₀, e.g. a force of 200kilonewtons (kN), is being applied, the contact zone is preferablyapproximately in the range 20% of the total area of each of the firstsurfaces 28′, 29′ of the wedges 28, 29 to 50% of said total area.

In accordance with a characteristic of the invention, the domed orprotuberant central portion 30, 31 may include the center C of the firstsurface 28′, 29′ as the center of said central portion 30, 31 so thatthe protrusion of the domed portion 30, 31 coincides with thegeometrical center of the first surface 28′, 29′ of the wedge 28, 29,but it is also possible to make provision for said domed or protuberantcentral portion 30, 31 to be offset slightly relative to the center C ofthe first surface 28′, 29′: such an embodiment is, for example, shown inFIG. 7 in which the domed or protuberant central portion 31 is offsetslightly relative to the center C of the first surface 28′, 29′ of eachof the wedges 28, 29, so that the protrusion or maximum height of thecentral portion 30, 31 does not coincide exactly with the geometricalcenter C of the first surface 28′, 29′ of each of the wedges 28, 29.This offset or this eccentricity of the protrusion of the domed portion30, 31 relative to the center C of each of the first surfaces 28′, 29′is relatively small and, by way of example, lies in the range 2 mm to 10mm for a wedge of length 80 mm. This eccentricity or offset of theprotrusion of the domed or protuberant central portion 30, 31 relativeto the center C of the first surface 28′, 29′ may thus lie in the range0% of the length of the wedge 28, 29 to 40% of the length thereof.

In a possible embodiment of the invention, the wedges 28, 29 areidentical, i.e. their dimensions are mutually equal and their firstsurfaces 28′, 29′ are mutually identical, both in shape and in size.

However, it is also quite possible, as described for the variousaccompanying figures, for the two wedges 28, 29 not to be identical,i.e. essentially for their first surfaces 28′, 29′ not to be the same,and optionally for only one of the first surfaces 28′, 29′ of saidwedges 28, 29 to have a domed or protuberant central portion 30, 31.

1. A press brake for bending at least one metal sheet, said press brakecomprising: an upper table having a bottom edge carrying first bendingtools, and a lower table having a top edge carrying second bendingtools, the two tables being movable relative to each other to exert abending force on the sheet; said press brake presenting a verticalmidplane, one of said tables having, through the entire thicknessthereof, two slots disposed symmetrically about the midplane, each slothaving a first edge and a second edge, and an open first end opening outin a side edge of the table, as well as a closed end; at least onestopper being disposed in each slot, each stopper comprising a firstwedge having a first end secured to the first slot edge and a second endforming a first surface, and a second wedge having a first end securedto the second slot edge and a second end forming a first surface; andthe first surface of at least one of the first and second wedges havinga central portion that is protuberant relative to the other portions ofsaid surface so that the contact between the first and the second wedgesis established essentially over said central portion.
 2. A press brakeaccording to claim 1, wherein said central portion of at least one ofthe first and second wedges is domed.
 3. A brake press according toclaim 1, wherein the first surface of the first wedge and the firstsurface of the second wedge both have respective central portions thatare protuberant relative to the other portions of said first surfaces.4. A press brake according to claim 3, wherein said respective centralportions are domed.
 5. A press brake according to claim 1, wherein thefirst surface of the first wedge and/or the first surface of the secondwedge is a convex surface.
 6. A press brake according to claim 1,wherein the first surface of one of the wedges presents a concavesurface while the first surface of the other wedge presents a convexsurface.
 7. A press brake according to claim 1, wherein the firstsurface of at least the first wedge and/or of the second wedge is aspherical surface portion.
 8. A press brake according to claim 1,wherein, at least in the zone of the stoppers, the slots presentconstant height so that the first edge and the second edge are parallel,in the absence of bending force for bending the metal sheet.
 9. A pressbrake according to claim 8, wherein the first surfaces of the wedges areinclined relative to the parallel edges of the slots.
 10. A press brakeaccording to claim 9, wherein the first surfaces of the wedges areinclined at a slope lying in the range 1% to 40%, relative to an axisthat is parallel to the edges of the slots.
 11. A press brake accordingto claim 10, wherein said slope lies in the range 5% to 10% relative tothe axis that is parallel to the edges of the slots.
 12. A press brakeaccording to claim 1, wherein the first surface of at least one of thefirst wedge and the second wedge presents a plurality of inclined planeperipheral portions connecting to the central portion.
 13. A press brakeaccording to claim 1, wherein the central portion presents a protrudingheight lying in the range 0.05 mm to 0.25 mm relative to the otherportions of the first surface for a wedge of length substantially equalto 80 mm.
 14. A press brake according to claim 1, wherein the wedges aremounted on supports connected to respective ones of the edges of theslots.
 15. A press brake according to claim 14, wherein at least one ofthe supports is movable laterally, i.e. along an axis parallel to theparallel edges of the slot on which said movable support is mounted. 16.A press brake according to claim 1, wherein the two wedges are offsetrelative to each other laterally, i.e. along an axis parallel to theparallel edges of the slot.
 17. A press brake according to claim 1,wherein, in the absence of bending force for bending the metal sheet,the first and second wedges present clearance between them.
 18. A pressbrake according to claim 1, having a plurality of stoppers disposed inrespective ones of the slots symmetrically about the midplane.